In recent years, it is strongly demanded that internal combustion engines such as automotive internal combustion engines address low power consumptions and regulate carbon dioxide emissions and unburned gas emissions from the viewpoint of global environmental protection. In order to meet such a demand, new types of internal combustion engines such as lean-burn engines, direct-injection engines and low-emission engines have been developed. For ignition of air-fuel mixtures in these developed internal combustion engines and for efficient ignition of air-fuel mixtures in the conventional internal combustion engines, there is a need for spark plugs that have higher ignition performance than conventional ones.
As one type of such a high-ignition-performance spark plug, a spark plug is known which includes a center electrode, a noble metal tip welded to a front end of the center electrode, a ground electrode and a noble metal tip welded to a front end of the ground electrode facing the center electrode so as to generate a spark discharge between the noble metal tips.
Patent Document 1 discloses a spark plug that includes a center electrode (30) and a ground electrode (40) arranged facing each other, with a spark gap (50) left between the center and ground electrodes, and a noble metal tip (45) joined at one end thereof by laser welding to a spark-gap-side surface (43) of the ground electrode, wherein the cross sectional area of the other end of the noble metal tip is in the range of 0.12 mm2 to 1.55 mm2; wherein the length (L) of protrusion of the noble metal tip from the ground electrode is in the range of 0.3 mm to 1.5 mm; and wherein there is formed a fused part (47) in which the ground electrode and the noble metal tip are fused together so that the outer surface (47a) of the fused part linking the circumferential surface (45a) of the noble metal tip with the surface (43) of the ground electrode to which the noble metal tip is joined has a concave curved shape with a radius (R) of curvature of 0.1 mm to 1.0 mm.
The noble metal tips of the respective electrodes are formed of alloys containing noble metals such as platinum and iridium as main components. As these alloys are expensive, there is a problem that the spark plugs with the noble metal tips increase in cost.
Another type of spark plug in which the electrode has a protrusion formed by processing the ground electrode itself in place of the noble metal tip has also been proposed. For example, there is proposed a technique of forming a protrusion on one side of the ground electrode by extruding the opposite side of the ground electrode. More specifically, Non-Patent Document 1 mentions as follows: “This feature enables it to be pushed-out directly from top towards center electrode as shown in FIG. 16. By conducting this process, new fine-wired portion was achieved with smaller material cost and a more simple production way when compared to the conventional fine-fired electrode.”, that is, teaches that the ground electrode can be produced at low cost by extrusion process.